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KAlSi3O8 A colorless, white, cream-yellow, flesh-reddish, or gray potassium feldspar that usually contains some sodium feldspar, either as albite or analbite or in some intermediate state; it is or appears to be monoclinic. Also known as common feldspar; orthose; pegmatolite.



a rock-forming mineral of the feldspar group with the chemical composition K[AlSi3O8]. Orthoclase contains admixtures of Na (up to 8 percent Na2O); less frequently, it contains Ba and small amounts of Fe, Ca, Rb, and Cs. It crystallizes in the monoclinic system to form prismatic crystals. Twin crystals are characteristic. Orthoclase, unlike microcline, exhibits perfect cleavage at a 90° angle (hence the name “orthoclase”). It is light pink, brownish yellow, or red. The mineral has a vitreous luster, a hardness on Mohs’ scale of 6–6.5, and a density of 2,550–2,580 kg/m3.

Orthoclase is one of the principal rock-forming minerals in magmatic rocks. Clusters of large orthoclase crystals occur in pegmatite veins. The mineral is often formed during regional and contact metamorphism. Orthoclase is used in the manufacture of glassware and ceramics.